Ameren Seminar

August 19–20, 2008

Effingham, IL
SCR Catalyst Management

2008 WPCA/Ameren

ESP and SCR Seminar

Effingham, IL

August 20, 2008
Agenda

z SCR Catalyst Basics

z Background on SCR catalyst and design considerationsz Provides basis for relevance of Catalyst Management

z Catalyst Management
SCR Catalyst Basics
Quick Review of SCR Basics

z SCR = Selective Catalytic Reducz Purpose is to reduce NO

x (NO & 

combustion exhaust

z Ammonia (NH

3) is injected into fl

reducing agent. Flue gas passes catalyst layers installed in a react

z NH

3 reacts with NO

x on the catal

to form nitrogen and water vapor

CatalystCatalyst

4NO + 4NH

3 + O

2 4N

2 

2NO

2 + 4NH

3 + O

2 3N

2 

Flue Gas: NO

x, O2, SO2

N2, H2O, O2, SO2(SO3)

NH3
SCR Configuration in Power Plants

High Dustz SCR upstream of air preheater

and ESP

z High concentration of fly ash in 

exhaust

z Catalyst with higher pitch 

required

ESP

SCR Reactorwith Catalyst

Air

Preheater

Boiler

Air

Preheater

Hot-Side ESP

Low Dustz SCR between ESP and air 

preheater

z Low concentration of fly ashz Catalyst with smaller pitch can 

be suitable
Catalyst Types

Plate Honeycomb Corrugated
Plate-type Catalyst

Compositionz Stainless steel carrier, ce

material rolled on

z TiO2, V-oxide/W-oxide/Mo

the active catalytic materi

z Notches (corrugations) fo

plates to provide separati

z Inserted in element boxes 

variable spacing: 60 to 90 

z Variable plate height: 450 

Advantages

z Ideal for high dust configuz Plugging resistancez Low pressure loss
Honeycomb Catalyst

Compositionz Homogeneously extruded ceram

square-opening cell structure

z TiO2, V-oxide and W-oxide as

the active catalytic material

z Variable block height: 1200+ mmz 8 to 14 cpsi for coal-fired applica

Advantages

z Ideal for low/no dust applicationz High active surface area per un
pitch pitch

Pitch = center to center distance from one plate/wall to the nex

Catalyst Pitch

Plate-Type Structurez Flexible platesz Rectangular Openingsz Pitch: 5 mm to 7 mm

Honeycomb Structure

z Rigid structurez Square openingsz Pitch: 6.7 mm to 9.2 m
Pitch Selection vs Dust Load

>12 6.5 mm NA

10 – 12 6.2 mm 9.2 mm (16 Cell)

<2 5.0 mm 6.7 mm (22 Cell)

6 – 10 6.0 mm 8.2 mm (18 Cell)

2 – 6 5.5 mm 7.4 mm (20 Cell)

Honeycomb

Pitch

Plate

Pitch

Dust Load

gr/dscf
Catalyst Modules

z Catalyst elements arranged in s

z Plate – 2 levels of 8 element z Honeycomb – 72 monoliths

z Standardized cross-sectionz Possible to interchange catalys

reactor

z Module height varies with catal
Catalyst Design Considerations

z SCR Reactor

z Initial catalyst charge or reloadz Reactor size – layers, modules per layerz Plant configuration – high dust / low dust

z Flue Gas Operating Conditions

z Operating Temperaturez Fuel Characteristicsz Fly ash concentration

z Performance Requirements

z Amount of NO

x to reduce and operating life

z NH

3 slip allowed, 2 ppm standard 

z SO

2 oxidation allowed

z Pressure drop limit, < 1.0” wc per layer
Required Reactor Potential

z Minimum catalytic potential required to achieve desired DeN

NH

3 slip rates for given operating period

z Catalyst deactivates with time, k

0

Î

kt (time at end of life)

z Fresh catalyst activity, k

0,depends on

Catalyst formulation 

Î

V

2

O

5

TemperatureFlue gas composition – H

2O, O

2, SO

2

z Iterative process to determine required AV and k 

RP =

kt

AV

AV =

Vfg

Volcat *A

kt = activity at time t (EOL)

AV = area velocity
Masking:Macroscopic blockage of catalyst surface by cemented fly ash

Plugging:Microsopic blockage of pore systemby small fly ash particles

Poisoning:Deactivation of sites by chemic

Catalyst Surface

Pore System

Small Fly Ash Particles

Catalyst Surface

Pore System

Cemented Fly Ash

Catalyst Surfa

Pore System

Catalyst Deactivation Mechanisms
Catalyst Poisons

z Arsenic – vapor phase As

2

O

3

z Phosphorus – P

2

O

5

z Alkali metals – Na, K
Relative Activity, kt/k0

z Initially (t=0), kt/k0 = 1.0z kt/k0 decreases with time (exponentially decaying model) z Characteristic of SCR configuration, fuel, deactivation 

mechanisms.

z Obtained from catalyst activity monitoringz Have kt Î k0 or have k0 Î kt

Lignite firing 16000 0.50 - 0.55

PRB firing 16000 0.65 - 0.70

Bituminous firing 16000 0.70 - 0.75

Low dust config 16000 0.80 - 0.85

Application time, hr kt/k0
Effects of Sulfur in Fuel

z SO2 oxidation, undesired side reaction in SCR catalyst

2SO2 + O2 2SO3

z Rate depends on T and catalyst formulationz Occurs in the bulk mass of catalyst material

SO3/H2SO4 aerosols cause the “Blue Plume”

z SO3 reacts with NH3 to form ammonium bisulfate (ABS) and 

sulfate (AS)NH3 + SO3 + H2O NH4HSO4 (ABS)

NH4HSO4 + NH3 (NH4)2SO4 (AS)

2NH3 + SO3 + H2O (NH4)2SO4 (AS)

z AS is dry, powdery compoundz ABS is sticky, viscous compound that can plug catalyst and 

equipment
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

RelativeActivity

V2O5 content [wt-%]

012345

Relative

Oxidation Rate

kSOx

kNOx

SO

2 Oxidation – Influence of V

2

O

5
Minimum Operating Temperature, (Tmin

)

z Minimum temperature for NH

3

injection

z Of particular concern for low load 

operation and firing fuels containing sulfur

z Operating above Tmin prevents 

AS/ABS formation

z Tmin depends on SO

3, NH

3, H

2O 

in flue gas 

z Must stop NH

3 injection when 

operating below Tmin

Tmin vs SO3 inlet

NOx in 250 ppm, 88% DeNOx, NH3 

525550575600625

0 5 10 15 20 

SO3 inlet (ppm wet) 

Tmin (F) 
Catalyst Management
Catalyst Management Goals

z Maintain target NO

x reduction of the SCR system

z Control NH

3 slip below required limit

z Replace activity potential lost to catalyst deactivationz Optimize use of catalyst activity potentialz Conform catalyst replacements to plant outage schedulez Conform to plant budgetary constraintsz Maximize operating life between catalyst replacementsz Prevent excessive pressure loss
Catalyst Management Plan (CMP)

z Schedule for maintaining SCR catalytic potential for NO

x red

z Process of adding, replacing, cleaning catalystz Tailored to fit unit performance based on activity monitoring
Relative ReactorPotential

Operating Time (h)

0 20,000 40,000 60,000 80,000 100,000

0.20.40.81.00.6

NH3 slip(ppm)

CMP, 2 + 1 Example

Minimum 

DeNO

requ
Activity Monitoring

z Routine catalyst activity testingz Once per year (typically)z Pull samples from each installed catalyst layerz Determine relative activity compared to fresh catalystz Determine rate of catalyst deactivationz Predict when reactor potential reaches minimum levelz Verify or adjust CMP schedule
CMP Actions

z Install new catalystz Onsite (in situ) Cleaning – removing fly ash/LPA buildup

z Vacuumingz Air blowingz Shaking/vibrationz Water washing

z Rejuvenation – onsite or offsite washing/chemical treatment 

z Remove ash/particles from catalyst pores z Remove water soluble poisons

z Regeneration – offsite chemical treatment

z Remove chemically bonded poisons – As, Pz Replace active catalytic material
Pros/Cons for CMP Options

Done offsite, may necatalystMay increase SO

2 c

Performance guaran

Lower cost than new catalystCan restore to “new” activity

Regeneration

(Service Supplier)

Does not restore to activity, limited life ePerformance guaran

Lower cost than new catalystCan be done onsite

Rejuvenation

(Service Supplier)

Lowest activity restoLimited extension of 

Low(est) cost optionCan be done in situ

Cleaning

(by Owner)

Highest Cost Option Longest operating life Performance guarantees New Catalyst (Catalyst Supplier)

CMP Action Pros Cons
Considerations for CMP Options

z CMPs are unit-specificz Economic Trade-offs: New vs Regen

z Lower cost for Regen, shorter operating lifez Higher cost for New, longer operating lifez Spare catalyst layer on-hand may be required to replac

removed for regen

z Catalyst supplier offers performance guarantees for new z Does regen supplier offer guarantees for Regen’d catal

z Consider which options conform best to plant outage schedu

z Cleaning/rejuvenation may not extend catalyst life to nez Consider operation with lower DeNO

x, higher NH

3 slip

z Consider impact to plant emission standards
Catalyst Mixing

z Standardized module structure – catalyst mixing is possible 

z Multiple catalyst types in reactorz New and regen’dz Multiple suppliers

z Consider tooling requirements

z Lifting, transport tooling for unloading/loadingz Sealing systems

z Performance Guarantees from new supplier (other than initia

z Layer guarantees – geometry, durability, activity, SO2 c

pressure drop – based on fuel and flue gas characterist

z System guarantees – DeNOx, NH3 slip, operating life – 

reactor potential Æ requires info about existing catalyst
Required Info for Existing Catalyst

z Geometry

z Catalyst typez Volumez Specific area, m

2/m

3

z Activity Trends

z Fresh catalyst, initial activity – k

0

z Relative/absolute activity after exposure to flue gasz Age of catalyst at time of testing

z Above info used to . . .

z Calculate existing reactor potentialz Determine rate of deactivationz Develop CMP
Plant Operation Considerations

z Control Ash buildup

z Sootblowers/Sonic hornsz Cleaning/vacuuming during outages

z Uniform NH

3/NO

x distribution

z AIG Tuningz Maintain NH

3 injection nozzles

z Limit injecting NH

3 below Tmin

z Short time operation OK (< 8 hr, 100 hr/yr)z Period below Tmin must be followed by higher Temp operation 

hours)

z Effects of Operating changes on SCR performance

z Fuel changesz Economizer modification
Thank You!

Ken JeffersApplication Engineer, SCR Catalystken.jeffers@argillon.com678 341 7523Argillon LLC5895 Shiloh Road, Suite 101Alpharetta, GA 30005www.argillon.com